Tropical Cyclone Winston is bearing down on the main island of Fiji with local authorities warning of life-threatening conditions from destructive winds, torrential rain and storm surges.

At 5 a.m. local time on Saturday the Category 4 storm was 320 km (199 miles) east-northeast of the Fijian capital Suva and producing average wind speeds of 220 km per hour.

Fiji's official meteorological service warned residents in the Pacific island nation's east to "expect very destructive hurricane-force winds."

"The potential for devastation is high and there is a very real risk that people will lose their lives," Neville Koop of the Na Draki Weather Service in Suva warned.

Winston had earlier tracked south and west of Fiji, causing extensive damage on the Vava'u island group in Tonga before moving on to a collision course with Fiji earlier this week.

Shipping within 300 nautical miles of Winston has been asked to report every three hours and to expect "phenomenal seas within 30 nautical miles of the centre."

Meanwhile, Virgin Airlines on Saturday morning announced all scheduled flights in and out of the Fijian city of Nadi would be suspended this weekend. Monday's schedule was under review.